The thalamus of the monotremes: cyto- and myeloarchitecture and chemical neuroanatomy

Abstract

Echidna and platypus brains were sectioned and stained by Nissl or myelin stains or immunocytochemically for calcium-binding proteins, gamma aminobutyric acid (GABA) or other antigens. Cyto-and myeloarchitecture revealed thalami that are fundamentally mammalian in organization, with the three principal divisions of the thalamus (epithalamus, dorsal thalamus and ventral thalamus) identifiable as in marsupials and eutherian mammals. The dorsal thalamus exhibits more nuclear parcellation than hitherto described, but lack of an internal medullary lamina, caused by splaying out of afferent fibre tracts that contribute to it in other mammals, makes identification of anterior, medial and intralaminar nuclear groups difficult. Differentiation of the ventral nuclei is evident with the ventral posterior nucleus of the platypus enormously expanded into the interior of the cerebral hemisphere, where it adopts a relationship to the striatum not seen in other mammals. Other nuclei such as the lateral dorsal become identifiable by expression of patterns of calciumbinding proteins identical to those found in other mammals. GABA cells are present in the ventral and dorsal thalamic nuclei, and in the ventral thalamus form a remarkable continuum with GABA cells of the two segments of the globus pallidus and pars reticulata of the substantia nigra.

Thalamic nuclei in the opossum Monodelphis domestica

“…Nevertheless, there are also many parallels in the structure of the thalamus between these species. The topological relations of the main groups of thalamic nuclei in Monodelphis and monotremes (as revealed by immunocytochemistry and other stainings) are very similar (Ashwell and Paxinos, 2005;Mikula et al, 2007). As in other species, anatomy of the epithalamus is especially conservative.…”

“…Presence of a clear internal medullary lamina make discerning of the anterior and medial groups of thalamic nuclei in the opossum easy, while its supposed absence in monotremes (Mikula et al, 2007) makes the task much harder. The complex of the ventral nuclei that is so expanded in monotremes, is only modestly developed in Monodelphis, signaling lesser importance of the somatosensory system in that species.…”

“…In monotremes cytoarchitectonic borders of many thalamic nuclei are indistinct and some of them may be revealed only with immunohistolchemical labelings (cf. Ashwell and Paxinos, 2005;Mikula et al, 2007). Comparisons of the brains of extant species of monotremes and other mammals are very complicated because both platypus and echidna have highly specialized sensory systems and in each species these specializations are different.…”

“…that are present in the thalamus of monotremes is not fully clear but it was to a large extent clarified with immunocytochemical labelings (Ashwell and Paxinos, 2005;Mikula et al, 2007). The thalamus of the eutherian and marsupial mammals has many common features, but in both suborders there is a wide spectrum of evolutionary derivations.…”

Thalamic Reticular Nucleus in <b><i>Caiman crocodilus</i></b>: Immunohistochemical Staining

“…Despite some observations to the contrary [Spreafico et al, 1991;Clemence and Mitrofanis, 1992;Nakatani, 1993], the morphology and orientation of these neurons in the thalamic reticular nucleus are considered to represent a single population of neurons. Even so, this may not preclude the thalamic reticular nucleus from being subdivided, as has been suggested for certain mammals [Mikula et al, 2008]. Neurons in the thalamic reticular nucleus are oriented parallel to the external medullary lamina and perpendicular to the fibers in the internal capsule [Cajal, 1966;Scheibel and Scheibel, 1966;Yen et al, 1985;Ohara and Havton, 1996].…”